Ultrasonic transducers and imaging systems are used in many medical applications and, in particular, for the non-invasive acquisition of images of organs and conditions within a patient, typical examples being the ultrasound imaging of fetuses and the heart. Such systems commonly use a linear or phased array transducer having multiple transmitting and receiving elements to transmit and receive narrowly focused and "steer able" beams, or "lines", of ultrasonic energy into and from the body. The transmitted beams, or lines, are reflected from the body's internal structures as received beams, or lines, that contain information that is used to generate images of the body's internal structures.
In a typical application, such as cardiac scanning, a number of beams or lines are transmitted and received along a plurality of angles forming a sector, that is, a wedge shaped three dimensional volume of interest, wherein the angular width of a sector may be the full range of angles that the transducer is capable of generating and receiving, or a selected portion of that range. The lines of a sector are typically organized into "frames" wherein each frame contains data representing a volume of interest, that is, a sector, and may be further processed or viewed to extract or present the information of interest.
The sequence and timing in which the lines are acquired and the organization of lines into frames often depends upon the particular application and the information desired and is affected by such factors as the dynamics of the information that is being acquired, the time required to transmit and receive a line, the data processing necessary to extract the desired information, and the processing required to generate an image display of the information. For example, in certain types of cardiac scanning the frames may be organized so that each frame contains data representing the sector at a selected point in time in the cardiac cycle so that the dynamic operation of the heart in a volume of interest can then be observed by viewing successive frames.
In many applications, however, these requirements conflict or interact to produce undesirable results. For example, one important application of ultrasonic imaging is color flow mapping wherein doppler information is extracted from the returning signals to generate images, or maps, of blood flow velocity in, for example, the chambers of a heart. Color flow mapping, however, requires multiple data acquisitions, typically 8 to 12 along each line, and the time required for each acquisition along a line is determined by the speed of ultrasound wave in the body and the maximum depth of the volume of interest from the transducer. As a result, one or all of the frame rate, that is, the rate at which data is acquired, the line density, that is, the granularity or sharpness of the map as determined by the number of lines used to generate the map, or the field of view, that is, the angular width and depth of the sector as determined by the number and length of the lines, are compromised.
The ultrasound imaging systems of the prior art, including color flow mapping systems, have addressed this problem in a number of ways, such as allowing the systems to be configured in operate in either or both of the "rapid burst" and "interleaved line" modes.
In the "rapid burst" mode, the system transmits and receives a sequence or set of lines along a single direction, wherein the set of lines along a single direction is referred to as a "packet", and this process is repeated across the sector so that the set of all of the packets of the sector comprise a frame. That is, and to illustrate by a simple example, in a given system each packet may be comprised of four lines in the same direction, so that the lines and packets may be designated as lines A1, A2, A3 and A4 in the A packet, each of which are directed at an angle A, lines B1, B2, B3 and B4 in the B packet, which are directed at an angle B, lines C1, C2, C3 and C4 in the C packet, each of which are directed at an angle C, and so on. A frame is then comprised of all of the lines of the packets A, B, C and so on, while a sector is the total angle of view covered by angles A, B, C, and so on. In the instance of a "rapid burst" system, the lines are transmitted and received in the sequence A1, A2, A3, A4, B1, B2, B3, B4, C1, C2, C3, C4, and so on. Because all of the lines in a given direction, that is, along a given angle, are transmitted and received in a rapid sequence, the "rapid burst" method is advantageous in applications where the condition being observed is changing rapidly, such as in blood flow mapping of regions wherein the blood is fast flowing or wherein it is necessary to identify and map relatively short transients in the blood flow. In this instance, a frame represents a relatively small interval in time with respect to the cardiac cycle, but is sufficient in time to show blood movement.
In the "interleaved line mode", the total set of lines per frame is the same as in the rapid burst method but the sequence of transmission and reception of the lines is in a different order from the rapid burst method in that the system interleaves the acquisition of lines in a pattern among a sequence of two or more packets. Illustrating the "interleaved line mode" by means of the same example as just discussed for the rapid burst method, the sequence of transmission and reception of lines could be, for example, A1, B1, C1, A2, B2, C2, A3, B3, C3, and so on. As a consequence, interleaving reduces the pulse repetition rate in each direction, that is, the rate at which lines are transmitted and received in a given direction, but does so while acquiring the same total number lines in the same interval as does the rapid burst mode. This approach may be used, for example, where the condition being observed is relatively stable or changes relatively slowly, such as in blood flow mapping in regions where blood is moving relatively slowly. In such situations longer sampling times are necessary in order to observe blood movement and interleaving allows more time for the blood to move between "looks" in each direction while not affecting the frame rate.
A significant limitation of systems requiring multiple acquisitions along each line, however, such as in blood flow mapping systems or in B-mode systems operating with multiple transmit focus depths, is that the requirement for multiple acquisitions along each line limits the number of lines that can be acquired in an allowable time, even using "rapid burst" or "interleaved line" operation, thereby limiting the data acquisition rate of the system and, for example, the system resolution.
More recent blood flow mapping systems have therefore used a "parallel line" method wherein each transmission and reception is comprised of a single transmitted line and multiple received lines and wherein the term "parallel" refers to the simultaneous or concurrent reception of multiple lines. The received lines are typically offset to either side of but straddling the transmitted line, thereby increasing the density of acquired lines and, as a consequence, the resolution of the acquired data. This method may be used in conjunction with the "rapid burst" or "interleaved line" modes of operation and, as presently implemented, typically provides two or four received lines for each transmitted line, referred to respectively as two way and four way parallel receive systems.
The parallel line method of operation, however, is susceptible to "parallel artifacts", which appear in the displayed image as periodic gain variations in the lateral direction across the image, that is, as strong, weak, strong, weak, and so on, variations in the image. While the variation can be slight, for example on the order of 1 dB, the variation is very noticeable in the image because it is periodic and because it is fixed, relative to the to transducer, rather than moving with the tissues or fluids being imaged.
Parallel artifacts are typically caused by asymmetric shading of the transducer aperture and a consequent asymmetric variation in the transmitted signal strength by, for example, a rib or an air bubble. The asymmetric shading of the transducer aperture, in turn, causes an apparent shift in the origin of the transmit lines on the aperture while the foci of the transmitted lines remain in their correct, original position in space, thereby causing the transmit lines to effectively pivot slightly around their foci. The receive lines, however, are continuously focused in the typical system and are thereby not affected and do not pivot around their foci. The round trip gain of a line signal increases when the transmit directions are closer to the receive directions and decreases when the transmit directions are farther from the receive directions, so that when the transmit lines pivot around their foci there is a resulting periodic strong/weak parallel artifact that swaps from strong to weak and the reverse at the transmit focal depth.
There are yet other parallel artifacts which arise because the direction of the round-trip path of a transmitted and received line is the combination of and lies between the transmit and receive directions. Typically, the receive lines are over-steered to direct the round-trip lines in the desired directions, but the parallel round-trip beam shapes still have mirror symmetry rather than translational symmetry. As a consequence, a target or region forming an image or part of an image that moves laterally with a constant velocity moves through the image in a slightly non-uniform way, somewhat like an inchworm, and causes a moving image to appear as if it was viewed through rippled glass. As with the first described parallel artifact, this is very noticeable because it is periodic and does not move along with the tissues or fluids being imaged.
The systems of the prior art using multiple, parallel received lines have attempted to eliminate or reduce the affects of parallel artifacts by the use of lateral spatial filters with equal weighting for odd and even lines to attenuate the artifacts. As is well understood in the art, spatial filtering utilizes periodicity of data in space, such as provided by the evenly spaced received lines of a scanning ultrasound transducer, to attenuate unwanted affects at one point in space by, in effect, averaging the data at that one point in space with data acquired from one or more periodically adjacent points in space. This approach has been somewhat successful in systems using two way parallel receive systems, but the method effectively reduces the resolution of the data and results in degraded or blurred images. While the results of spatial filtering are undesirable but acceptable for many applications in two way parallel receive systems, the degradation and blurring of the image in four way parallel receive systems is unacceptable.
The present invention provides a solution to these and other problems of the prior art.